1. Field of the Invention
The present invention relates to MOS transistors and manufacturing process thereof, and more particularly, to trench-typed power MOS transistors and manufacturing process thereof.
2. Description of the Related Art
Power MOS transistors are a specific type of MOS transistor used for providing and switching power in an integrated circuit. Accordingly, power MOS transistors must be able to work normally under high voltage. Therefore, it is typical for a power MOS transistor cell manufactured by CMOS process to be of a large size in order to be capable of operation under high voltage. On the other hand, it is also a requirement that power MOS transistors be able to output large current. Therefore, it is also typical to combine a large number of power MOS transistor cells into a single power MOS transistor, wherein each power MOS transistor cell outputs a relatively small amount of current. However, such a power MOS transistor must be very large, which may be unacceptable in view of manufacturing requirements nowadays.
To reduce the size of power MOS transistors, a vertical diffused MOS (VDMOS) transistor is introduced. FIG. 1 shows a schematic view of a VDMOS transistor. Unlike traditional planar CMOS transistors, the current flows vertically in a VDMOS transistor. As shown in FIG. 1, the source region is on the top of the VDMOS transistor 100, and the drain region is on the bottom of the VDMOS transistor 100. Such structure enables the VDMOS transistor 100 both a high breakdown voltage and a high current output.
FIG. 2 shows a schematic view of another type of trench typed MOS transistor, i.e. a UMOS transistor. The name “UMOS” derives from the U-shaped gate oxide of the UMOS transistor 200. As shown in FIG. 2, the UMOS transistor 200 has a trench-shaped gate extending into the silicon, and the current also flows vertically in a UMOS transistor. Similar to VDMOS transistors, the source region is on the top of the UMOS transistor 200, and the drain region is on the bottom of the UMOS transistor 200.
However, since VDMOS and UMOS transistors both adopt a vertical structure, they cannot be manufactured or integrated with other logic circuits on a CMOS-processed chip. As a result, the manufacturing of an integrate circuit utilizes either VDMOS transistors or UMOS transistors as power MOS transistors is both complicated and expensive. Accordingly, there is a need to design a power MOS transistor that not only can maintain a high breakdown voltage, high output current, high operation speed, but also adopts a horizontal structure such that it can be integrated with other logic circuits on a CMOS-processed chip.